[Synthacon VCF]

[Magnus Danielson]

From: "tomg" <efm3 at mediaone.net>
Subject: Re: [Synthacon VCF]
Date: Mon, 4 Dec 2000 19:30:09 -0800

> Thank you Mike and Romeo...hummmm...Diode ring?

Yes, thanks to your both...

Um, the bottom 2N5172s obviously looks like a standard expo-curcuit.
I assume that the signal-in three-position switch will select the
characteristic. It is not too uncommon to do such a thing by inserting
the signal at diffrent places. I assume that this filter is a 12 db/Oct
filter, right?

The diode directions of the right 2N5172 seems kind of wrong at first,
but if you redraw the schematics with +10 Vdc at top, 1 k (splitt off
for a 2k2 to the left 2N5172), a 2k2, two diodes, a cap tap, one
diode, a cap tap, one diode, a cap, two diodes and then into the
collector of the left 2N5172. Now, the right 2N5172 will pull current
out of it's 2k2 resistor in parallell with the diode ladder (which is
really a single sided ladder and not a double sided as the EMS). Thus,
the current out of that 2k2 is being divided into the diode ladder and
the expo transistors collector. The middle capacitor tap is really the
output of the diode-core and the 2N5172 and 2N5138 is wired up as a
amplifier. The output of that is then feeded into the diode-core
through two 2nF caps at the two other cap-taps. You select
characteristics by inserting signal either at the middle cap-tap
(really the input of the amp), at the lower cap tap or at the top of
the common current feed.

This initial analysis could possibly help people to get some grip of
it. It's just my random thougths as I look over the schematic.

Cheers,
Magnus